User equipment (UE), also known as mobile stations, wireless terminals and/or mobile terminals are enabled to communicate wirelessly in a wireless communication network, sometimes also referred to as a cellular radio system. The communication may be made e.g. between two user equipment units, between a user equipment and a regular telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks.
The user equipment units may further be referred to as mobile telephones, cellular telephones, laptops with wireless capability. The user equipment units in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another user equipment or a server.
The wireless communication system covers a geographical area which is divided into cell areas, with each cell area being served by a network node, or base station e.g. a Radio Base Station (RBS), which in some networks may be referred to as “eNB”, “eNodeB”, “NodeB” or “B node”, depending on the technology and terminology used. The network nodes may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the network node/base station at a base station site. One base station, situated on the base station site, may serve one or several cells. The network nodes communicate over the air interface operating on radio frequencies with the user equipment units within range of the respective network node.
In some radio access networks, several network nodes may be connected, e.g. by landlines or microwave, to a Radio Network Controller (RNC) e.g. in Universal Mobile Telecommunications System (UMTS). The RNC, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural network nodes connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipment units. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
In the present context, the expressions downlink, downstream link or forward link may be used for the transmission path from the network node to the user equipment. The expression uplink, upstream link or reverse link may be used for the transmission path in the opposite direction i.e. from the user equipment to the network node.
Functionality to handle user mobility is a fundamental component in wireless communication systems. From a service quality perspective, such functionality must ensure that service continuity is maintained as user equipments move from one cell to another during an active session, and that each new session is established in a sufficiently good radio environment. From a spectral efficiency perspective, such functionality should ensure that an active user is always served by the most appropriate base station or base stations, which typically means the closest base station/s in a radio sense.
Macro diversity through soft handover has proven to be a key feature of CDMA-based cellular networks such as WCDMA. As specified by 3GPP, WCDMA is a wireless multiple access protocol based on code division multiple access, distributed Signal-to-Interference-Ratio (SIR) based power control operating at 1500 Hz, frequency reuse one, (partially) non orthogonal data and control channels.
Effective interference control is a must for high performing WCDMA networks, not only to maximize achievable data rates and capacity, but also to achieve stable control channel performance—which in turn is needed to meet end user and operator expectations on service continuity and retainability (measured e.g. as dropped call rate). In this context, soft handover is a key method to cope with high interference, poor coverage, and user mobility.
Soft handover, or soft handoff as it also may be referred to, refers within the present context to a feature used within some wireless communication networks based on CDMA/WCDMA standards, where a user equipment is simultaneously connected to two or more cells (or cell sectors) during a call/connection.
More specifically, soft handover serves as a means to achieve interference control, robust “make before break” handovers, and diversity with respect to fading radio channels caused by multi-path interference and shadowing effects.
In WCDMA according to 3GPP Release 99 as well as later enhancements such as High Speed Downlink Packet access (HSDPA) and High Speed Uplink Packet access (HSUPA), mobility management is handled by the Radio Network Controller (RNC). The RNC determines on the fly which cells should constitute the active set: i.e., the cells that a user equipment is connected to simultaneously in a soft handover fashion. To help the RNC prioritize between candidate cells, the user equipment estimates the signal quality or signal strength of the Primary Common Pilot Channel (P-CPICH) transmitted on the downlink in each cell. The UE reports this information to the RNC in an event-triggered or periodic manner.
Soft handover is supported for Dedicated Channels (as of 3GPP Rel-99) and for HSUPA (as of 3GPP Rel-6). However, to simplify the network architecture soft handover was not introduced for the High Speed Downlink Shared Channel (HS-DSCH), High Speed Shared Control Channel (HS-SCCH) and High Speed Dedicated Physical Control Channel (HS-DPCCH) in HSDPA (Rel-5).
The macro diversity functionality for the downlink dedicated channels relies on synchronous (time aligned) transmissions over the Uu interface (i.e., over the air). In the user equipment, signals received from all cells that are currently part of the active set are combined in the receiver. By time aligning the transmissions, a minimum of buffering is required in the user equipment, thereby reducing complexity and improving performance.
The radio access network has been designed to facilitate synchronous transmission in different radio links, transmitted from the same or different base stations (or NodeBs).
This behaviour is accomplished by an interaction of the radio network controller, which hosts the Radio Link Control (RLC) protocol, and the base stations, as illustrated in FIG. 1. A frame control over the lub interface (connecting the NodeB and RNC) comprises tight timing adjustment, such that downlink data frames are transmitted within a certain restricted window of time. The connection between the NodeB and the RNC is also referred to as backhaul link. Further limited buffering in the NodeB then enables data frames to be transmitted at exactly the same time over the air, for compensating for jitter etc.
Soft handover also has other merits, for example to handle imbalances of downlink and uplink path loss with respect to adjacent cells. Especially in heterogeneous networks, composed of base stations of different output power, it is likely that the best cell with respect to the uplink and the downlink are different. To maximize downlink and uplink channel quality, e.g. represented by the SIR, the user equipment would need to connect to different base stations in each link. This is because, as illustrated in FIG. 2: the uplink path gain (inverse of path loss) and interference level at the base station receiver will determine the best server in the uplink, and the downlink Ec/N0 (which is dependent on base station powers, path gain, and downlink interference level) will determine the best server in the downlink.
However, by means of soft handover, an optimum handover decision can be done for downlink while keeping the best possible uplink radio link as part of the active set. Hence the achievable data rate and signalling quality for the user of interest can be maximized simultaneously for downlink and uplink, respectively.
Another virtue of soft handover exploited for the uplink is Inter-Cell Interference Control (ICIC), which relies on: Soft Handover (SHO) in conjunction with inner-loop power control. These mechanisms ensure that the user equipment utilizes a transmit power so that the SIR target is met only for the strongest radio link.
Further, the HSUPA overload indicator (relative grant). For HSUPA connections, this may be sent from the non-serving Node-B on the Enhanced Relative Grant Channel (E-RGCH). By means of the overload indicator a non-serving cell may command the user equipment to reduce its grant.
Having tight interference control and “make before break” handovers means the mobility mechanisms do not need to be as precise. Hence they allow for relaxed requirements on accuracy of mobility measurements in the user equipment and ensure robust system performance in dynamic radio environments.
Observe that in the sequel the terms lub, backhaul, and transmission are used interchangeably to describe the connection between RNC and NodeB.
To be able to attach a NodeB via an IP network to an RNC is an interesting concept. Thereby it may be possible to attach for example a femto cell to the network in a convenient manner. However, the prolonged signal propagation time that would result from the backhaul signalling over the IP network makes soft handover difficult to implement. If links of the active set belonging to different NodeBs are subject to different backhaul transmission delays, which could be the case especially over best effort IP transport networks, all links of an active set need to await the worst possible link. I.e., it is the base station having the maximum possible backhaul delay that determines the point of transmission over the air interface in all cells. Higher delay means degraded service quality, larger buffering requirements, and decreased node capacity.
Therefore, in a traditional WCDMA system, strict requirements on backhaul delay characteristics have been employed. The tolerated average delay and jitter (delay variation) were determined to achieve reasonable buffer size and a low impact on service quality, primarily driven by the generally accepted perception of adequate speech quality.
Current lub frame protocol synchronization procedures are thus relying on relatively low variations in delay, and the maximum difference between links in the active set may not be too high. This reliance is not compatible with the generally looser delay characteristics of best effort IP transport networks.
Static adjustments to the looser characteristics in accordance with prior art techniques are not appealing for the reasons of service quality and node complexity described above.